How Much Secondary Containment Capacity Is Required
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How Much Secondary Containment Capacity Is Required?
Ask three engineers how much containment a tank needs and you will hear three numbers, all of them quoted with confidence. The confusion is understandable: the rule is written differently in different jurisdictions, it changes depending on how many tanks share a compound, and it is nearly always stated without the freeboard and rainwater conditions that make it work in practice.
The underlying principle is constant - the containment must hold the contents of the largest single container, plus allowance for precipitation - and the common European formulation adds a second test based on total volume. Center Enamel designs containment to whichever is governing, and issues the calculation with the tank package so the permit application has a defensible number behind it.
1. What Does the 110 Percent Rule Actually Mean?
The containment must hold at least the full volume of the largest container, and in most formulations an additional margin - giving the familiar 110 percent figure - plus enough freeboard to contain rainfall.
· Largest Container Test: The governing case is the largest single tank in the compound, not the sum of all tanks.
· The Additional Margin: The extra ten percent covers uneven bases, residual liquid and the practical impossibility of building a perfectly level bund floor.
· Precipitation Allowance: US SPCC rules require additional capacity for rainfall and snowmelt; ignoring it is the most common way a compliant-looking bund fails an inspection.
· European Formulation: Commonly expressed as 110 percent of the largest container or 25 percent of the total volume of all containers, whichever is greater.
2. How Do the Main Rules Compare?
The tests differ in detail but converge on the same outcome for a typical single-tank installation; the differences appear as soon as several tanks share a compound.
· US EPA SPCC (40 CFR 112): Contain the capacity of the largest single container plus sufficient freeboard for precipitation, with provisions for draining rainwater under controlled conditions.
· UK and EU Practice: Bund capacity of at least 110 percent of the largest container, or 25 percent of the aggregate volume of all containers, whichever gives the larger figure.
· Double Wall Tanks: Where the tank itself provides secondary containment, the annulus or outer shell is sized on the same principle, which can remove the bund requirement subject to the permit.
· Local Permit Conditions: Environmental permits frequently impose stricter figures, particularly in groundwater protection zones; the permit always governs.
3. What Reduces the Required Volume?
Two legitimate routes: interlocked tanks counted as a single volume, and integral double wall containment that removes the bund from the calculation.
· Interlocked Tanks: Tanks permanently interconnected so they behave as one volume can be assessed as a single container rather than several, subject to the authority's acceptance.
· Integral Double Wall Design: Where the outer shell provides full containment, the civil bund may be reduced or removed - but only with the regulator's agreement in writing.
· Level and Overflow Control: Reliable overfill protection reduces the credible release volume and is often a permit condition in its own right.
· Drained and Covered Compounds: A roofed, drained compound removes the precipitation allowance, though the drain must discharge to a safe, controlled point.
Rule or Jurisdiction | Requirement | Practical Note |
US EPA SPCC, 40 CFR 112 | Largest container plus precipitation freeboard | Rainwater must be drainable under controlled conditions |
UK and EU practice | 110% of largest container or 25% of aggregate, whichever greater | The aggregate test governs in large compounds |
Double wall tanks | Outer shell sized to full primary volume | May remove the bund subject to permit |
Groundwater protection zones | Frequently stricter than the baseline | Permit condition always governs |
Interlocked tank banks | Assessed as one container if permanently connected | Requires the authority's acceptance |
Engineering Assurance and Project Support
Every tank delivered by Shijiazhuang Zhengzhong Technology Co., Ltd. (Center Enamel) is engineered against AWWA D103-09 and EN 1090 with finite element verification of shell, roof and nozzle loads, fused at 820-930°C under ISO 9001 and ISO 45001 control, holiday tested at 1500 V across one hundred percent of the surface, and assembled with Grade 8.8 bolts and manufacturer-certified sealant. Containment calculations are issued with the tank package, including the governing test, the precipitation allowance and the freeboard assumption, so the permit submission rests on a documented calculation.
Frequently Asked Questions (FAQ)
What is the 110 percent rule for bunds?
The bund must hold 110 percent of the volume of the largest container within it - the full contents plus a ten percent margin - and in US SPCC terms also provide freeboard for precipitation.
Does rainwater count against bund capacity?
Yes in practice. US SPCC rules require additional capacity or a controlled drainage arrangement for rainfall and snowmelt, because a bund half full of rainwater has already lost half its containment.
How do you size containment for multiple tanks?
Apply both tests: 110 percent of the largest container, and - under UK and EU practice - 25 percent of the aggregate volume of all containers. Whichever figure is greater governs.
Does a double wall tank still need a bund?
Often not, because the tank provides its own secondary containment. However, the exemption depends on the local regulator and the permit conditions, so written agreement should be obtained before deleting the bund from the design.
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